Under iron-limited conditions, Pseudomonas putida WCS358 produces a siderophore, pseudobactin 358, which is essential for the plant growth-stimulating abiity of this strain. Cells of strain WCS358, provided that they have been grown under Fe3+ limitation, take up 55Fe3+ from the 5sFe3 -labeled pseudobactin 358 complex with Km and V,, values of 0.23 ,uM and 0.14 nmol/mg of cell dry weight per min, respectively. Uptake experiments with cells treated with variods metabolic inhibitors showed that this Fe3+ uptake process was dependent on the proton motive force. Furthermore, strain WCS358 was shown to be able to take up Fe3+ complexed to the siderophore of another plant-beneficial P. fluorescens strain, WCS374. The tested pathogenic rhizobacteria and rhizofungi were neither able to grow on Fe3+-defitcient medium in the presence of pseudobactin 358 nor able to take up "5Fe3+ from 55Fe3 -pseudobactin 358. The same applies for three cyanide-producing Pseudomonas strains which are supposed to be representatives of the minor pathogens. These results indicate that the extraordinary ability of strain WCS358 to compete efficiently for Fe3+ is based on the fact that the pathogenic and deleterious rhizosphere microorganisms, in contrast to strain WCS358 itself, are not able to take up Fe3+ from Fe3+-pseudobactin 358 complexes.Frequent cultivation of monocultures on the sarne field is a practical demand of modern agriculture. However, frequent cultivation of, e.g;, potato in the same field results in yield decreases of up to 30% (11,12,22). The causal agents of these yield decreases are assumed to be deleterious, cyanide-producing Pseudomonas spp. (1, 23). The rhizosphere also harbors various pathogenic microorganisms which influence the potato yield, e.g., bacteria like Erwinia carotovora, which can cause rotting of the potato tubers, and fungi like Verticillium spp., which may cause wilting of the potato plants (12,23).Bacterization of seed potatoes with certain fluorescent Pseudomonas spp. has a beneficial effect on potato yield (2, 11). These plant-beneficial Pseudomonas strains have been selected after screening of large numbers of fluorescent, root-colonizing Pseudomonas spp. on antibiosis activity against a series of rhizosphere microorganisms (10). For some Pseudomonas spp. this antibiosis activity is primarily based on the production of antibiotic compounds (5), while for other Pseudomonas spp., like Pseudomonas putida WCS358, antibiosis is based on successful competition for Fe3" by strain WCS358 in comparison with that by the pathogenic or deleterious microorganisms (9,22,23). Under Fe3" limitation, the beneficial Pseudomonas cells produce powerful fluorescent siderophores (7,17,18,25), Fe3+-chelating compounds, which are part of high-affinity Fe3+ uptake systems. Recently, it has been demonstrated that these beneficial Pseudomonas strains actually produce these siderophores in the rhizosphere (3,23). Also the ability of the beneficial Pseudomonas strain to produce siderophores was shown to be a prerequisite for the...